Rhodium complexes are extremely useful as catalysts in commercial processes for the production of many organic chemicals. DE-C1-26 27 354 describes hydroformylation wherein catalytically active rhodium complexes containing water-soluble sulfonated aryl phosphines are used as ligands.
Since rhodium catalysts are expensive, the economic visability of processes of the foregoing type depends upon substantially complete recovery of the rhodium from catalysts which have become inactive. As is generally known, use of such catalysts over a substantial period of time will reduce both their activity and selectivity. Contaminants in the reactants, chemical transformations of the complexes, reactions of the complexes with the reaction products, etc. all contribute to the irreversible poisoning and/or inactivation of the catalysts. Hence, it is necessary to take appropriate steps to regenerate these catalysts and return them to service in the reactions. In the past, the rhodium has been recovered and transformed into the catalytic derivative.
German Application P 36 26 536.5 teaches a method for recovering rhodium from such aqueous solutions. An excess based on the rhodium of a carboxylic acid having 7 to 22 carbon atoms is added to the complex-containing solution. An oxidizer, such as oxygen or air, is introduced into the mixture at 50.degree. to 200.degree. C. The rhodium precipitates as an insoluble compound and is separated thereby. Using this method, 90% or more of the original rhodium is recovered commercially.